The copolymer of isobutylene and isoprene, commonly known as butyl rubber (IIR) and the halobutyl rubber (HIIR) derived from butyl rubber by addition of bromine or chlorine, are extensively used in several applications, being the tire manufacture the major.
In the prior art several inventions have been applied to the design of the polymerization reactor for the production of the butyl rubber.
U.S. Ser. No. 448,575, filed Jun. 26, 1942 then abandoned, and CA 463453 both by John H. Bannon may be considered among the first applications on the concept of the tubular polymerization reactor. In this reactor a draft tube provided with an agitator is arranged centrally of the reactor and a plurality of return tubes are arranged between headers and around the central draft tube, with means for circulating a suitable refrigerant, desirably vaporizing ethylene, through the space between the headers and around the central draft tube as well as the return tubes.
A back-mixed reactor is employed; typically a one-tube pass system as described by reference to U.S. Pat. No. 2,474,592. Such reactor consists of a vertical vessel formed by an enclosing side wall within which is provided an axially mounted draft tube of relatively large diameter surrounded by large number of small diameter tubes which extend downwardly. An axial flow pump, which extends into the draft tube is located in the bottom of the reactor to pump the reaction mixture up the draft tube. The reaction mixture includes the diluents, catalyst and reactants, which are directly introduced into the bottom of the reactor, and a portion of the reaction mixture which after up flowing through the draft tube is recycled from the top of the reactor downwardly through the tubes which surround the draft tube. The outer walls of the reaction vessel form a jacket through which a liquid hydrocarbon coolant is circulated to remove the exothermic heat of reaction via heat exchange contact with the outer walls of the small diameter tubes, and wall of the central draft tube.
U.S. Pat. No. 2,577,856 and its continuation-in-part application U.S. Pat. No. 2,636,026 represent a different version of the vertical shell and tube reactor, in which the central draft tube provided with an impeller is surrounded, rather than by several smaller tubes, by concentric annular surfaces, continually withdrawing heat from the annular walls by indirect heat transfer with a coolant circulating in one or more annular chambers. The second patent describes a serious problem encountered in each commercial tubular reactor, due to accumulation of polymer on the upper entrance tube sheet and plugging of the return tubes, especially at their entrance ends; the trouble is general and persistent under widely varying conditions of operation.
U.S. Pat. No. 2,999,084 concerns the use of the vertical tubular reactor with the central draft tube, wherein the improvement comprises the injection of the feed solution containing isobutylene/isoprene in methyl chloride at a zone of higher stream velocity immediately below the propeller. The patent refers that in commercial experience the mass fouling is a limiting factor of prime importance with respect to the rate of production of butyl rubber polymer; fouling inhibits adequate refrigeration and is the reason of the operation of the reactor in run having intervals within the range of about 10 to 90 hours; cleaning out the reactor before resuming the polymerization reaction normally requires 10 to 20 hours.
European patent EP0053585 proposes a different type of polymerization reactor suitable for the production of butyl rubber at a temperature ranging from −40 to −110° C. The reactor incorporates an internal rotary coolant chamber, a coaxial agitator and an external cooling jacket. The outer and inner faces of the rotary chamber are kept clean by rotating scrapers.
Even if this type of reactor can keep the heat exchange surfaces free from rubber deposits, it has the disadvantage of a limited exchange surface and therefore of a limited productivity.
The European patent EP0209253 describes a still fully different polymerization reactor and process, in which the monomers mixture of isobutylene and isoprene together with a polymerization medium, consisting in a mixture of halogenated and not halogenated hydrocarbons, is polymerized in a self cleaning screw extruder at a temperature of −50 to +15° C., somewhat higher than prior art close to −100° C. Heat of polymerization is removed by evaporative cooling of the reaction medium. Even if of academic interest, this application, due to the high reaction temperature, is not able to produce butyl rubber acceptable by the market and it has not been used industrially.
The Russian patents RU1615935 and RU2097122 propose a reactor applicable for butyl rubber, wherein the arrangement of the polymerization slurry and the cooling medium is the opposite compared with the conventional vertical tubular reactor with central draft tube: the polymerization reaction occurs in the shell of the vessel, supplied with a central multiple blades stirrer, while the refrigerating ethylene passes through vertical tube bundles (four in the patent drawings) peripherally disposed and put in from the upper head of the reactor. A drawback of this invention is the high asymmetric arrangement, due to the position of the tube bundles, and the highly not homogeneous velocity of the rubber slurry inside the reactor vessel.
In spite of the alternative reactor arrangements described in the above mentioned and in other patents, as reported in the Ullmann's-Encyclopedia of Industrial Chemistry-fifth edition, the most used industrial reactor corresponds to the vertical tubular reactor with draft tube model, as proposed in the original U.S. Ser. No. 448,575 and better defined in U.S. Pat. No. 2,474,592 and U.S. Pat. No. 2,999,084.
Whereas this reactor has been commercially used by the industry for many years for conducting these types of reactions, the reactor is less efficient than desirable.
Effective agitation is of particular importance in that the polymerization reaction is exothermic and in that the molecular weight of the polymer product is adversely affected by increases in temperature. Thus, when the reaction medium is not of an entirely homogeneous composition, localized overheating may occur, resulting in the formation of undesirable polymeric materials which adhere tenaciously to metal surfaces within the reaction vessel. This phenomenon, commonly referred to as mass fouling, has presented a problem with respect to the production of butyl rubber.
Moreover, a gradual and uniform buildup of polymer deposits upon and fouls the heat transfer surfaces within the reaction vessel; the polymer adheres to the metal surfaces as a continuous film. Nevertheless, polymer fouling presents a problem and, in consideration of the downtime to remove the fouling, it has limited the efficiency of this type of reactor, as also reported in the above mentioned U.S. Pat. No. 2,999,084.
In U.S. Pat. No. 5,417,930, Exxon describes a new model of butyl rubber tubular reactor, without the draft tube and having a reduced fouling tendency compared with the older conventional type of reactor with central draft tube, as described in U.S. Pat. No. 2,999,084 and in other patents.
The reactor contains a two-tubes pass system, constituted of an inner or center tube bundle, through which a mixture or slurry of polymerizable monomers, diluent and catalyst is passed in one direction and recycled via an outer tube bundle in the opposite direction in essentially even flow distribution.
The tubular bundles are maintained within a jacketed section, where a refrigerant removes the exothermic heat of reaction from the polymerization mixture and maintains the polymerization mixture at uniformly low temperature. An even flow circulation of the slurry, which aids in maintaining uniform low temperature, is provided by the use of a diffuser and mixed flow pumping system.
The proposed model is more complex than the conventional reactor and further subject of limiting factors.
In fact while it may be agreed in principle that the flow distribution in the inner tube bundle will be uniform, such uniformity does not appear to be granted for what concerns the flow distribution in the outer tube bundle.
Furthermore the pressure drop in the two-tube pass design will be higher compared with the conventional simple-pass design.
These and other considerations show that there is a need for better design of a single pass reactor for the production of the butyl rubber.